1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission and, more particularly, to a hydraulic control system using a fewer number of solenoid valves to control the friction elements of the automatic transmission.
2. Description of Related Art
In conventional hydraulic control systems for automatic transmissions having a plurality of friction elements such as clutches and brakes, these hydraulic control systems include a friction element controller. The friction element controller includes a plurality of solenoid valves. Specifically, the friction element controller includes one solenoid valve corresponding to each friction element. Each solenoid valve controls application and release of the corresponding brake or clutch (i.e. friction element).
By being able to independently control each friction element, the simultaneous engagement of two friction elements, which would cause the automatic transmission to fail, is prevented. For instance, one well-known automatic transmission includes first, second and third clutches and first, second, third and fourth brakes. Simultaneously engaging both the first and second brakes, both the second clutch and third brake, or both the third clutch and the fourth brake, will cause the automatic transmission to fail. By providing a solenoid value corresponding to each friction element, for independent control thereof, in the hydraulic control system, failure of the automatic transmission because of a detrimental simultaneous engagement of friction elements can be prevented.
Because of their cost, however, having such a large number of solenoid valves significantly increases the cost of the hydraulic control system.